diff --git a/core/config/schema/core.data_types.schema.yml b/core/config/schema/core.data_types.schema.yml
index 763853c1b46..f43dbb50543 100644
--- a/core/config/schema/core.data_types.schema.yml
+++ b/core/config/schema/core.data_types.schema.yml
@@ -62,6 +62,15 @@ label:
   type: string
   label: 'Label'
   translatable: true
+  constraints:
+    Regex:
+      # Forbid any kind of line ending:
+      # - Windows: `\r\n`
+      # - old macOS: `\r`
+      # - *nix: `\n`
+      pattern: '/(\r\n|\r|\n)/'
+      match: false
+      message: 'Labels are not allowed to span multiple lines.'
 
 # String containing plural variants, separated by EXT.
 plural_label:
diff --git a/core/modules/block/tests/src/Kernel/BlockValidationTest.php b/core/modules/block/tests/src/Kernel/BlockValidationTest.php
index c055f29203d..f9acd355bdd 100644
--- a/core/modules/block/tests/src/Kernel/BlockValidationTest.php
+++ b/core/modules/block/tests/src/Kernel/BlockValidationTest.php
@@ -3,6 +3,7 @@
 namespace Drupal\Tests\block\Kernel;
 
 use Drupal\block\Entity\Block;
+use Drupal\Core\Config\Entity\ConfigEntityInterface;
 use Drupal\KernelTests\Core\Config\ConfigEntityValidationTestBase;
 
 /**
@@ -27,6 +28,9 @@ class BlockValidationTest extends ConfigEntityValidationTestBase {
       'id' => 'test_block',
       'theme' => 'stark',
       'plugin' => 'system_powered_by_block',
+      'settings' => [
+        'label' => 'Powered by Drupal 🚀',
+      ],
     ]);
     $this->entity->save();
   }
@@ -62,4 +66,27 @@ class BlockValidationTest extends ConfigEntityValidationTestBase {
     return $cases;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  protected static function setLabel(ConfigEntityInterface $block, string $label): void {
+    static::assertInstanceOf(Block::class, $block);
+    $settings = $block->get('settings');
+    static::assertNotEmpty($settings['label']);
+    $settings['label'] = $label;
+    $block->set('settings', $settings);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function testLabelValidation(): void {
+    static::setLabel($this->entity, "Multi\nLine");
+    // TRICKY: because the Block config entity type does not specify a `label`
+    // key, it is impossible for the generic ::testLabelValidation()
+    // implementation in the base class to know at which property to expect a
+    // validation error. Hence it is hardcoded in this case.
+    $this->assertValidationErrors(['settings.label' => "Labels are not allowed to span multiple lines."]);
+  }
+
 }
diff --git a/core/modules/contact/config/schema/contact.schema.yml b/core/modules/contact/config/schema/contact.schema.yml
index 5fb69d63393..7e9fd9fee4e 100644
--- a/core/modules/contact/config/schema/contact.schema.yml
+++ b/core/modules/contact/config/schema/contact.schema.yml
@@ -28,7 +28,7 @@ contact.form.*:
       type: integer
       label: 'Weight'
     message:
-      type: label
+      type: text
       label: 'Message displayed to user on submission'
     redirect:
       type: path
diff --git a/core/modules/contact/tests/src/Kernel/ContactFormValidationTest.php b/core/modules/contact/tests/src/Kernel/ContactFormValidationTest.php
index b84b2ff626b..f07571c142d 100644
--- a/core/modules/contact/tests/src/Kernel/ContactFormValidationTest.php
+++ b/core/modules/contact/tests/src/Kernel/ContactFormValidationTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\contact\Kernel;
 
+use Drupal\contact\ContactFormInterface;
 use Drupal\contact\Entity\ContactForm;
 use Drupal\KernelTests\Core\Config\ConfigEntityValidationTestBase;
 
@@ -30,4 +31,14 @@ class ContactFormValidationTest extends ConfigEntityValidationTestBase {
     $this->entity->save();
   }
 
+  /**
+   * Tests validation of message.
+   */
+  public function testMessageValidation(): void {
+    assert($this->entity instanceof ContactFormInterface);
+    // Messages should be able to span multiple lines.
+    $this->entity->setMessage("Multi\nLine");
+    $this->assertValidationErrors([]);
+  }
+
 }
diff --git a/core/modules/editor/tests/src/Kernel/EditorValidationTest.php b/core/modules/editor/tests/src/Kernel/EditorValidationTest.php
index 670b7225c42..5c6b1b87e3a 100644
--- a/core/modules/editor/tests/src/Kernel/EditorValidationTest.php
+++ b/core/modules/editor/tests/src/Kernel/EditorValidationTest.php
@@ -70,4 +70,13 @@ class EditorValidationTest extends ConfigEntityValidationTestBase {
     $this->assertValidationErrors(['editor' => "The 'non_existent' plugin does not exist."]);
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function testLabelValidation(): void {
+    // @todo Remove this override in https://www.drupal.org/i/3231354. The label of Editor entities is dynamically computed: it's retrieved from the associated FilterFormat entity. That issue will change this.
+    // @see \Drupal\editor\Entity\Editor::label()
+    $this->markTestSkipped();
+  }
+
 }
diff --git a/core/modules/field/tests/src/Kernel/Entity/FieldConfigValidationTest.php b/core/modules/field/tests/src/Kernel/Entity/FieldConfigValidationTest.php
index fde68011cea..c4c4709e119 100644
--- a/core/modules/field/tests/src/Kernel/Entity/FieldConfigValidationTest.php
+++ b/core/modules/field/tests/src/Kernel/Entity/FieldConfigValidationTest.php
@@ -3,6 +3,7 @@
 namespace Drupal\Tests\field\Kernel\Entity;
 
 use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
 
 /**
  * Tests validation of field_config entities.
@@ -53,4 +54,36 @@ class FieldConfigValidationTest extends FieldStorageConfigValidationTest {
     ]);
   }
 
+  /**
+   * Tests validation of a field_config's default value.
+   */
+  public function testMultilineTextFieldDefaultValue(): void {
+    // First, create a field storage for which a complex default value exists.
+    $this->enableModules(['text']);
+    $text_field_storage_config = FieldStorageConfig::create([
+      'type' => 'text_with_summary',
+      'field_name' => 'novel',
+      'entity_type' => 'user',
+    ]);
+    $text_field_storage_config->save();
+
+    $this->entity = FieldConfig::create([
+      'field_storage' => $text_field_storage_config,
+      'bundle' => 'user',
+      'default_value' => [
+        0 => [
+          'value' => "Multi\nLine",
+          'summary' => '',
+          'format' => 'basic_html',
+        ],
+      ],
+      'dependencies' => [
+        'config' => [
+          $text_field_storage_config->getConfigDependencyName(),
+        ],
+      ],
+    ]);
+    $this->assertValidationErrors([]);
+  }
+
 }
diff --git a/core/modules/field/tests/src/Kernel/Entity/FieldStorageConfigValidationTest.php b/core/modules/field/tests/src/Kernel/Entity/FieldStorageConfigValidationTest.php
index 95ba5ec42bd..3ffbd029204 100644
--- a/core/modules/field/tests/src/Kernel/Entity/FieldStorageConfigValidationTest.php
+++ b/core/modules/field/tests/src/Kernel/Entity/FieldStorageConfigValidationTest.php
@@ -22,6 +22,7 @@ class FieldStorageConfigValidationTest extends ConfigEntityValidationTestBase {
    */
   protected function setUp(): void {
     parent::setUp();
+    $this->installEntitySchema('user');
 
     $this->entity = FieldStorageConfig::create([
       'type' => 'boolean',
diff --git a/core/modules/language/tests/src/Kernel/ContentLanguageSettingsValidationTest.php b/core/modules/language/tests/src/Kernel/ContentLanguageSettingsValidationTest.php
index 01a2e6a77ed..07bab877aaf 100644
--- a/core/modules/language/tests/src/Kernel/ContentLanguageSettingsValidationTest.php
+++ b/core/modules/language/tests/src/Kernel/ContentLanguageSettingsValidationTest.php
@@ -17,6 +17,11 @@ class ContentLanguageSettingsValidationTest extends ConfigEntityValidationTestBa
    */
   protected static $modules = ['language', 'user'];
 
+  /**
+   * {@inheritdoc}
+   */
+  protected bool $hasLabel = FALSE;
+
   /**
    * {@inheritdoc}
    */
diff --git a/core/modules/layout_builder/tests/src/Kernel/LayoutBuilderEntityViewDisplayValidationTest.php b/core/modules/layout_builder/tests/src/Kernel/LayoutBuilderEntityViewDisplayValidationTest.php
index 0c6032afb6a..740b531cc47 100644
--- a/core/modules/layout_builder/tests/src/Kernel/LayoutBuilderEntityViewDisplayValidationTest.php
+++ b/core/modules/layout_builder/tests/src/Kernel/LayoutBuilderEntityViewDisplayValidationTest.php
@@ -39,4 +39,13 @@ class LayoutBuilderEntityViewDisplayValidationTest extends ConfigEntityValidatio
     $this->entity->save();
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function testLabelValidation(): void {
+    // @todo Remove this override in https://www.drupal.org/i/2939931. The label of Layout Builder's EntityViewDisplay override is computed dynamically, that issue will change this.
+    // @see \Drupal\layout_builder\Entity\LayoutBuilderEntityViewDisplay::label()
+    $this->markTestSkipped();
+  }
+
 }
diff --git a/core/modules/rest/tests/src/Kernel/Entity/RestResourceConfigValidationTest.php b/core/modules/rest/tests/src/Kernel/Entity/RestResourceConfigValidationTest.php
index 0d8cb96272c..1859afa5dd5 100644
--- a/core/modules/rest/tests/src/Kernel/Entity/RestResourceConfigValidationTest.php
+++ b/core/modules/rest/tests/src/Kernel/Entity/RestResourceConfigValidationTest.php
@@ -18,6 +18,11 @@ class RestResourceConfigValidationTest extends ConfigEntityValidationTestBase {
    */
   protected static $modules = ['rest', 'serialization'];
 
+  /**
+   * {@inheritdoc}
+   */
+  protected bool $hasLabel = FALSE;
+
   /**
    * {@inheritdoc}
    */
diff --git a/core/modules/text/config/schema/text.schema.yml b/core/modules/text/config/schema/text.schema.yml
index e0fa5d3d83d..bd9cdb09cc0 100644
--- a/core/modules/text/config/schema/text.schema.yml
+++ b/core/modules/text/config/schema/text.schema.yml
@@ -138,7 +138,7 @@ field.widget.settings.text_textarea_with_summary:
       type: integer
       label: 'Number of summary rows'
     placeholder:
-      type: label
+      type: text
       label: 'Placeholder'
     show_summary:
       type: boolean
diff --git a/core/tests/Drupal/KernelTests/Core/Config/ConfigEntityValidationTestBase.php b/core/tests/Drupal/KernelTests/Core/Config/ConfigEntityValidationTestBase.php
index c0cf4928c68..aeed2a6737c 100644
--- a/core/tests/Drupal/KernelTests/Core/Config/ConfigEntityValidationTestBase.php
+++ b/core/tests/Drupal/KernelTests/Core/Config/ConfigEntityValidationTestBase.php
@@ -30,6 +30,20 @@ abstract class ConfigEntityValidationTestBase extends KernelTestBase {
    */
   protected ConfigEntityInterface $entity;
 
+  /**
+   * Whether a config entity of this type has a label.
+   *
+   * Most config entity types ensure their entities have a label. But a few do
+   * not, typically highly abstract/very low level config entities without a
+   * strong UI presence. For example: REST resource configuration entities and
+   * entity view displays.
+   *
+   * @see \Drupal\Core\Entity\EntityInterface::label()
+   *
+   * @var bool
+   */
+  protected bool $hasLabel = TRUE;
+
   /**
    * {@inheritdoc}
    */
@@ -288,6 +302,44 @@ abstract class ConfigEntityValidationTestBase extends KernelTestBase {
     $this->assertValidationErrors($expected_enforced_messages);
   }
 
+  /**
+   * Tests validation of config entity's label.
+   *
+   * @see \Drupal\Core\Entity\EntityInterface::label()
+   * @see \Drupal\Core\Entity\EntityBase::label()
+   */
+  public function testLabelValidation(): void {
+    // Some entity types do not have a label.
+    if (!$this->hasLabel) {
+      $this->markTestSkipped();
+    }
+    if ($this->entity->getEntityType()->getKey('label') === $this->entity->getEntityType()->getKey('id')) {
+      $this->markTestSkipped('This entity type uses the ID as the label; an entity without a label is hence impossible.');
+    }
+
+    static::setLabel($this->entity, "Multi\nLine");
+    $this->assertValidationErrors([$this->entity->getEntityType()->getKey('label') => "Labels are not allowed to span multiple lines."]);
+  }
+
+  /**
+   * Sets the label of the given config entity.
+   *
+   * @param \Drupal\Core\Config\Entity\ConfigEntityInterface $entity
+   *   The config entity to modify.
+   * @param string $label
+   *   The label to set.
+   *
+   * @see ::testLabelValidation()
+   */
+  protected static function setLabel(ConfigEntityInterface $entity, string $label): void {
+    $label_property = $entity->getEntityType()->getKey('label');
+    if ($label_property === FALSE) {
+      throw new \LogicException(sprintf('Override %s to allow testing a %s without a label.', __METHOD__, (string) $entity->getEntityType()->getSingularLabel()));
+    }
+
+    $entity->set($label_property, $label);
+  }
+
   /**
    * Asserts a set of validation errors is raised when the entity is validated.
    *
diff --git a/core/tests/Drupal/KernelTests/Core/Entity/EntityFormDisplayValidationTest.php b/core/tests/Drupal/KernelTests/Core/Entity/EntityFormDisplayValidationTest.php
index cda17605082..2b2eecdb2dc 100644
--- a/core/tests/Drupal/KernelTests/Core/Entity/EntityFormDisplayValidationTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Entity/EntityFormDisplayValidationTest.php
@@ -2,7 +2,10 @@
 
 namespace Drupal\KernelTests\Core\Entity;
 
+use Drupal\Core\Entity\Display\EntityFormDisplayInterface;
 use Drupal\Core\Entity\Entity\EntityFormDisplay;
+use Drupal\field\Entity\FieldConfig;
+use Drupal\field\Entity\FieldStorageConfig;
 
 /**
  * Tests validation of entity_form_display entities.
@@ -12,6 +15,11 @@ use Drupal\Core\Entity\Entity\EntityFormDisplay;
  */
 class EntityFormDisplayValidationTest extends EntityFormModeValidationTest {
 
+  /**
+   * {@inheritdoc}
+   */
+  protected bool $hasLabel = FALSE;
+
   /**
    * {@inheritdoc}
    */
@@ -19,7 +27,6 @@ class EntityFormDisplayValidationTest extends EntityFormModeValidationTest {
     parent::setUp();
 
     $this->entity = EntityFormDisplay::create([
-      'label' => 'Test',
       'targetEntityType' => 'user',
       'bundle' => 'user',
       // The mode was created by the parent class.
@@ -28,4 +35,45 @@ class EntityFormDisplayValidationTest extends EntityFormModeValidationTest {
     $this->entity->save();
   }
 
+  /**
+   * Tests validation of entity form display component's widget settings.
+   */
+  public function testMultilineTextFieldWidgetPlaceholder(): void {
+    // First, create a field for which widget settings exist.
+    $this->enableModules(['field', 'text']);
+    $text_field_storage_config = FieldStorageConfig::create([
+      'type' => 'text_with_summary',
+      'field_name' => 'novel',
+      'entity_type' => 'user',
+    ]);
+    $text_field_storage_config->save();
+
+    $text_field_config = FieldConfig::create([
+      'field_storage' => $text_field_storage_config,
+      'bundle' => 'user',
+      'dependencies' => [
+        'config' => [
+          $text_field_storage_config->getConfigDependencyName(),
+        ],
+      ],
+    ]);
+    $text_field_config->save();
+
+    // Then, configure a form display widget for this field.
+    assert($this->entity instanceof EntityFormDisplayInterface);
+    $this->entity->setComponent('novel', [
+      'type' => 'text_textarea_with_summary',
+      'region' => 'content',
+      'settings' => [
+        'rows' => 9,
+        'summary_rows' => 3,
+        'placeholder' => "Multi\nLine",
+        'show_summary' => FALSE,
+      ],
+      'third_party_settings' => [],
+    ]);
+
+    $this->assertValidationErrors([]);
+  }
+
 }
diff --git a/core/tests/Drupal/KernelTests/Core/Entity/EntityViewDisplayValidationTest.php b/core/tests/Drupal/KernelTests/Core/Entity/EntityViewDisplayValidationTest.php
index b30998db1ac..8b93ce26bab 100644
--- a/core/tests/Drupal/KernelTests/Core/Entity/EntityViewDisplayValidationTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Entity/EntityViewDisplayValidationTest.php
@@ -12,6 +12,11 @@ use Drupal\Core\Entity\Entity\EntityViewDisplay;
  */
 class EntityViewDisplayValidationTest extends EntityViewModeValidationTest {
 
+  /**
+   * {@inheritdoc}
+   */
+  protected bool $hasLabel = FALSE;
+
   /**
    * {@inheritdoc}
    */
@@ -19,7 +24,6 @@ class EntityViewDisplayValidationTest extends EntityViewModeValidationTest {
     parent::setUp();
 
     $this->entity = EntityViewDisplay::create([
-      'label' => 'Test',
       'targetEntityType' => 'user',
       'bundle' => 'user',
       // The mode was created by the parent class.